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Roles are a powerful way for designing complex behaviours while keeping good practices such as Single Responsibility, Dependency Inversion and Interface Segregation.
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/*
 * #%L
 * *********************************************************************************************************************
 *
 * These Foolish Things - Miscellaneous utilities
 * http://thesefoolishthings.tidalwave.it - git clone [email protected]:tidalwave/thesefoolishthings-src.git
 * %%
 * Copyright (C) 2009 - 2016 Tidalwave s.a.s. (http://tidalwave.it)
 * %%
 * *********************************************************************************************************************
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations under the License.
 *
 * *********************************************************************************************************************
 *
 * $Id$
 *
 * *********************************************************************************************************************
 * #L%
 */
package it.tidalwave.role;

import javax.annotation.CheckForNull;
import javax.annotation.Nonnull;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;
import java.util.ServiceLoader;
import it.tidalwave.util.NotFoundException;
import it.tidalwave.util.Task;
import it.tidalwave.role.spi.ContextManagerProvider;
import lombok.AccessLevel;
import lombok.NoArgsConstructor;
import lombok.extern.slf4j.Slf4j;

/***********************************************************************************************************************
 *
 * A facility to register and unregister global and local DCI contexts.
 *
 * @author  Fabrizio Giudici
 * @version $Id$
 *
 **********************************************************************************************************************/
public interface ContextManager
  {
    /*******************************************************************************************************************
     *
     * A locator for the {@link ContextManager} which uses the {@link ServiceProvider} facility to be independent of
     * any DI framework.
     *
     * This locator caches the internal reference and this is ok for production use; during tests, since multiple
     * contexts are typically created and destroyed for each test, you should call {@link #reset()} after each test
     * has been completed.
     *
     ******************************************************************************************************************/
    @Slf4j @NoArgsConstructor(access = AccessLevel.PRIVATE)
    public static final class Locator
      {
        @CheckForNull
        private static ContextManager contextManager;

        @CheckForNull
        private static ContextManagerProvider contextManagerProvider;

        @Nonnull
        public static synchronized ContextManager find()
          {
            if (contextManager == null)
              {
                if (contextManagerProvider == null)
                  {
                    final ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
                    final Iterator i =
                            ServiceLoader.load(ContextManagerProvider.class, classLoader).iterator();

                    if (!i.hasNext())
                      {
                        throw new RuntimeException("No ServiceProvider for ContextManagerProvider");
                      }

                    contextManagerProvider = Objects.requireNonNull(i.next());
                    log.trace("ContextManagerProvider instantiated from META-INF: {}", contextManagerProvider);
                  }

                contextManager = Objects.requireNonNull(contextManagerProvider.getContextManager(),
                                                        "Cannot find ContextManager");
              }

            return contextManager;
          }

        /** For testing. */
        public static void set (final @Nonnull ContextManagerProvider provider)
          {
            contextManager = null;
            contextManagerProvider = provider;
          }
      }

    /*******************************************************************************************************************
     *
     * Returns the list of current contexts, ordered by their priority.
     *
     * @return  the list of current contexts
     *
     ******************************************************************************************************************/
    @Nonnull
    public List